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Title: Br{sub 2} molecular elimination in 248 nm photolysis of CHBr{sub 2}Cl by using cavity ring-down absorption spectroscopy

Abstract

Elimination of molecular bromine is probed in the B {sup 3}{pi}{sub ou}{sup +}<-X {sup 1}{sigma}{sub g}{sup +} transition following photodissociation of CHBr{sub 2}Cl at 248 nm by using cavity ring-down absorption spectroscopy. The quantum yield for the Br{sub 2} elimination reaction is determined to be 0.05{+-}0.03. The nascent vibrational population ratio of Br{sub 2}(v=1)/Br{sub 2}(v=0) is obtained to be 0.5{+-}0.2. A supersonic beam of CHBr{sub 2}Cl is similarly photofragmented and the resulting Br atoms are monitored with a velocity map ion-imaging detection, yielding spatial anisotropy parameters of 1.5 and 1.1 with photolyzing wavelengths of 234 and 267 nm, respectively. The results justify that the excited state promoted by 248 nm should have an A{sup ''} symmetry. Nevertheless, when CHBr{sub 2}Cl is prepared in a supersonic molecular beam under a cold temperature, photofragmentation gives no Br{sub 2} detectable in a time-of-flight mass spectrometer. A plausible pathway via internal conversion is proposed with the aid of ab initio potential energy calculations. Temperature dependence measurements lend support to the proposed pathway. The production rates of Br{sub 2} between CHBr{sub 2}Cl and CH{sub 2}Br{sub 2} are also compared to examine the chlorine-substituted effect.

Authors:
; ; ; ; ; ;  [1];  [2]
  1. Department of Chemistry, National Taiwan University, Taipei 106, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan (China)
  2. (China)
Publication Date:
OSTI Identifier:
20868216
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 126; Journal Issue: 3; Other Information: DOI: 10.1063/1.2426334; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION SPECTROSCOPY; BROMINATED ALIPHATIC HYDROCARBONS; BROMINE; CHLORINATED ALIPHATIC HYDROCARBONS; DISSOCIATION; EXCITED STATES; MASS SPECTRA; MOLECULAR BEAMS; PHOTOCHEMISTRY; PHOTOLYSIS; PHOTON-MOLECULE COLLISIONS; POTENTIAL ENERGY; REACTION KINETICS; TEMPERATURE DEPENDENCE; TIME-OF-FLIGHT MASS SPECTROMETERS; TIME-OF-FLIGHT METHOD

Citation Formats

Wei Peiying, Chang Yuanping, Lee Yushan, Lee Weibin, Lin Kingchuen, Chen, K. T., Chang, A. H. H., and Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan. Br{sub 2} molecular elimination in 248 nm photolysis of CHBr{sub 2}Cl by using cavity ring-down absorption spectroscopy. United States: N. p., 2007. Web. doi:10.1063/1.2426334.
Wei Peiying, Chang Yuanping, Lee Yushan, Lee Weibin, Lin Kingchuen, Chen, K. T., Chang, A. H. H., & Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan. Br{sub 2} molecular elimination in 248 nm photolysis of CHBr{sub 2}Cl by using cavity ring-down absorption spectroscopy. United States. doi:10.1063/1.2426334.
Wei Peiying, Chang Yuanping, Lee Yushan, Lee Weibin, Lin Kingchuen, Chen, K. T., Chang, A. H. H., and Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan. Sun . "Br{sub 2} molecular elimination in 248 nm photolysis of CHBr{sub 2}Cl by using cavity ring-down absorption spectroscopy". United States. doi:10.1063/1.2426334.
@article{osti_20868216,
title = {Br{sub 2} molecular elimination in 248 nm photolysis of CHBr{sub 2}Cl by using cavity ring-down absorption spectroscopy},
author = {Wei Peiying and Chang Yuanping and Lee Yushan and Lee Weibin and Lin Kingchuen and Chen, K. T. and Chang, A. H. H. and Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan},
abstractNote = {Elimination of molecular bromine is probed in the B {sup 3}{pi}{sub ou}{sup +}<-X {sup 1}{sigma}{sub g}{sup +} transition following photodissociation of CHBr{sub 2}Cl at 248 nm by using cavity ring-down absorption spectroscopy. The quantum yield for the Br{sub 2} elimination reaction is determined to be 0.05{+-}0.03. The nascent vibrational population ratio of Br{sub 2}(v=1)/Br{sub 2}(v=0) is obtained to be 0.5{+-}0.2. A supersonic beam of CHBr{sub 2}Cl is similarly photofragmented and the resulting Br atoms are monitored with a velocity map ion-imaging detection, yielding spatial anisotropy parameters of 1.5 and 1.1 with photolyzing wavelengths of 234 and 267 nm, respectively. The results justify that the excited state promoted by 248 nm should have an A{sup ''} symmetry. Nevertheless, when CHBr{sub 2}Cl is prepared in a supersonic molecular beam under a cold temperature, photofragmentation gives no Br{sub 2} detectable in a time-of-flight mass spectrometer. A plausible pathway via internal conversion is proposed with the aid of ab initio potential energy calculations. Temperature dependence measurements lend support to the proposed pathway. The production rates of Br{sub 2} between CHBr{sub 2}Cl and CH{sub 2}Br{sub 2} are also compared to examine the chlorine-substituted effect.},
doi = {10.1063/1.2426334},
journal = {Journal of Chemical Physics},
number = 3,
volume = 126,
place = {United States},
year = {Sun Jan 21 00:00:00 EST 2007},
month = {Sun Jan 21 00:00:00 EST 2007}
}
  • Following photodissociation of CH{sub 2}Br{sub 2} at 248 nm, Br{sub 2} molecular elimination is detected by using a tunable laser beam, as crossed perpendicular to the photolyzing laser beam in a ring-down cell, probing the Br{sub 2} fragment in the B {sup 3}{pi}{sub ou}{sup +}-X {sup 1}{sigma}{sub g}{sup +} transition. The nascent vibrational population is obtained, yielding a population ratio of Br{sub 2}(v=1)/Br{sub 2}(v=0) to be 0.7{+-}0.2. The quantum yield for the Br{sub 2} elimination reaction is determined to be 0.2{+-}0.1. Nevertheless, when CH{sub 2}Br{sub 2} is prepared in a supersonic molecular beam under cold temperature, photofragmentation gives no Br{submore » 2} detectable in a time-of-flight mass spectrometer. With the aid of ab initio potential energy calculations, a plausible pathway is proposed. Upon excitation to the {sup 1}B{sub 1} or {sup 3}B{sub 1} state, C-Br bond elongation may change the molecular symmetry of C{sub s} and enhance the resultant 1 {sup 1,3}A{sup '}-X-tilde{sup 1}A{sup '} (or 1 {sup 1,3}B{sub 1}-X-tilde{sup 1}A{sub 1} as C{sub 2v} is used) coupling to facilitate the process of internal conversion, followed by asynchronous concerted photodissociation. Temperature dependence measurements lend support to the proposed pathway.« less
  • A primary dissociation channel of Br{sub 2} elimination is detected following a single-photon absorption of (COBr){sub 2} at 248 nm by using cavity ring-down absorption spectroscopy. The technique contains two laser beams propagating in a perpendicular configuration. The tunable laser beam along the axis of the ring-down cell probes the Br{sub 2} fragment in the B{sup 3}{Pi}{sub ou}{sup +}-X{sup 1}{Sigma}{sub g}{sup +} transition. The measurements of laser energy- and pressure-dependence and addition of a Br scavenger are further carried out to rule out the probability of Br{sub 2} contribution from a secondary reaction. By means of spectral simulation, the ratiomore » of nascent vibrational population for v = 0, 1, and 2 levels is evaluated to be 1:(0.65 {+-} 0.09):(0.34 {+-} 0.07), corresponding to a Boltzmann vibrational temperature of 893 {+-} 31 K. The quantum yield of the ground state Br{sub 2} elimination reaction is determined to be 0.11 {+-} 0.06. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state (COBr){sub 2} via internal conversion. A four-center dissociation mechanism is followed synchronously or sequentially yielding three fragments of Br{sub 2}+ 2CO. The resulting Br{sub 2} is anticipated to be vibrationally hot. The measurement of a positive temperature effect supports the proposed mechanism.« less
  • By using cavity ring-down absorption spectroscopy technique, we have observed the channel of Br{sub 2} molecular elimination following photodissociation of CF{sub 2}Br{sub 2} at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolyzing laser beam in a ring-down cell, is used to probe the Br{sub 2} fragment in the B {sup 3}{pi}{sub ou}{sup +}-X {sup 1}{sigma}{sub g}{sup +} transition. The vibrational population is obtained in a nascent state, despite ring-down time as long as 500-1000 ns. The population ratio of Br{sub 2}(v=1)/Br{sub 2}(v=0) is determined to be 0.4{+-}0.2, slightly larger than the value of 0.22 evaluatedmore » by Boltzmann distribution at room temperature. The quantum yield of the Br{sub 2} elimination reaction is also measured to be 0.04{+-}0.01. This work provides direct evidence to support molecular elimination occurring in the CF{sub 2}Br{sub 2} photodissociation and proposes a plausible pathway with the aid of ab initio potential-energy calculations. CF{sub 2}Br{sub 2} is excited probably to the {sup 1}B{sub 1} and {sup 3}B{sub 2} states at 248 nm. As the C-Br bond is elongated upon excitation, the coupling of the {sup 1}A{sup '}({sup 1}B{sub 1}) state to the high vibrational levels of the ground state X-tilde {sup 1}A{sup '}({sup 1}A{sub 1}) may be enhanced to facilitate the process of internal conversion. After transition, the highly vibrationally excited CF{sub 2}Br{sub 2} feasibly surpasses a transition barrier prior to decomposition. According to the ab initio calculations, the transition state structure tends to correlate with the intermediate state CF{sub 2}Br+Br(CF{sub 2}Br{center_dot}{center_dot}{center_dot}Br) and the products CF{sub 2}+Br{sub 2}. A sequential photodissociation pathway is thus favored. That is, a single C-Br bond breaks, and then the free-Br atom moves to form a Br-Br bond, followed by the Br{sub 2} elimination. The formed Br-Br bond distance in the transition state tends to approach equilibrium such that the Br{sub 2} fragment may be populated in cold vibrational distribution. Observation of a small vibrational population ratio of Br{sub 2}(v=1)/Br{sub 2}(v=0) agrees with the proposed mechanism.« less
  • The primary elimination channel of bromine molecule in one-photon dissociation of CH{sub 2}BrC(O)Br at 248 nm is investigated using cavity ring-down absorption spectroscopy. By means of spectral simulation, the ratio of nascent vibrational population in v = 0, 1, and 2 levels is evaluated to be 1:(0.5 {+-} 0.1):(0.2 {+-} 0.1), corresponding to a Boltzmann vibrational temperature of 581 {+-} 45 K. The quantum yield of the ground state Br{sub 2} elimination reaction is determined to be 0.24 {+-} 0.08. With the aid of ab initio potential energy calculations, the obtained Br{sub 2} fragments are anticipated to dissociate on themore » electronic ground state, yielding vibrationally hot Br{sub 2} products. The temperature-dependence measurements support the proposed pathway via internal conversion. For comparison, the Br{sub 2} yields are obtained analogously from CH{sub 3}CHBrC(O)Br and (CH{sub 3}){sub 2}CBrC(O)Br to be 0.03 and 0.06, respectively. The trend of Br{sub 2} yields among the three compounds is consistent with the branching ratio evaluation by Rice-Ramsperger-Kassel-Marcus method. However, the latter result for each molecule is smaller by an order of magnitude than the yield findings. A non-statistical pathway so-called roaming process might be an alternative to the Br{sub 2} production, and its contribution might account for the underestimate of the branching ratio calculations.« less
  • Following single-photon dissociation of CH{sub 2}I{sub 2} at 248 nm, I{sub 2} molecular elimination is detected by using cavity ring-down absorption spectroscopy. The technique comprises two laser beams propagating in a perpendicular configuration, in which a tunable laser beam along the axis of the ring-down cell probes the I{sub 2} fragment in the B {sup 3}{Pi}{sub ou}{sup +}- X {sup 1}{Sigma}{sub g}{sup +} transition. The nascent vibrational populations for v = 0, 1, and 2 levels are obtained with a population ratio of 1:(0.65 {+-} 0.10):(0.30 {+-} 0.05), corresponding to a Boltzmann-like vibrational temperature of 544 {+-} 73 K. Themore » quantum yield of the ground state I{sub 2} elimination reaction is determined to be 0.0040 {+-} 0.0025. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state CH{sub 2}I{sub 2} via internal conversion, followed by asynchronous three-center dissociation. A positive temperature effect supports the proposed mechanism.« less